The present invention generally relates to reproducing apparatuses for reproducing a pre-recorded unipolar signal, that is, a digital signal, from a recording medium such as a magnetic tape, and more particularly to a digital signal reproducing apparatus which is capable of carrying out accurate data discrimination upon reproduction by not only discriminating an amplitude information but also discriminating a bit period information on the time base.
As a known system for transmitting a digital signal, there is the partial response system. According to this partial response system, the transmission characteristic of the transmission path is taken into consideration, and a level detection is carried out with respect to a coded signal which has a code format (digital format) different from the code format cf the transmitted coded signal and is received in a satisfactory state. The code format of the received coded signal is restored back into the original code format of the transmitted coded signal.
On the other hand, when carrying out a magnetic recording and reproduction, there is a large deterioration in the response in the high frequency range during the recording and reproduction. In addition, because the reproducing system has a differential characteristic due to the winding of the magnetic head, there is a large attenuation in the low frequency components which are closer to D.C. components. Accordingly, there was a conventional magnetic recording and reproducing apparatus which employed the partial response system for the magnetic recording and reproduction of the digital signal. This conventional magnetic recording and reproducing apparatus recorded a digital signal having a code format which is matched with the magnetic recording and reproducing characteristics. Further, during the reproduction, the digital signals were reproduced by discriminating the level of the reproduced signal waveform, independent of the level fluctuations in the reproduced signal which were mainly caused by an unevenly coated magnetic layer, surface irregularities on the magnetic layer, and instability of the tape travel which are peculiar to the magnetic reproduction, and also independent of notable decrease in the level of the reproduced signal which is caused by signal dropouts due to dust particles and the like which are adhered on the magnetic surface.
According to the above conventional magnetic recording and reproducing apparatus, an analog audio signal which is to be recorded, for example, is subjected to a pulse code modulation (PCM) and modulated into a unipolar digital signal. The unipolar digital signal is passed through a converter and is then supplied to a 1-bit delay element (or a 2-bit delay element) wherein the unipolar digital signal is delayed by one bit transmission period. The delayed unipolar digital signal is fed back to the converter wherein the delayed unipolar digital signal is subjected to a modulo-2 addition (exclusive-OR operation) with a subsequent 1-bit input digital data, and is converted into a unipolar digital signal which is to be recorded. This unipolar digital signal which is obtained from the converter, is amplified in a recording amplifier, and is then recorded onto a magnetic tape by a recording magnetic head.
Next, in the reproducing system, the recorded unipolar digital signal is reproduced from the magnetic tape by a reproducing magnetic head. The reproduced signal has a pulse waveform which assumes a positive polarity when the recording current undergoes a transition from negative to positive polarity and assumes a negative polarity when the recording current undergoes a transition from positive to negative polarity, due to the differential characteristic of the reproducing system which is based on the characteristic of the winding of the reproducing magnetic head. The reproduced signal is amplified in a reproducing amplifier, and is then supplied to an equalizer. The equalizer compensates for the high frequency components which are attenuated during the process of the magnetic recording and reproduction. The equalizer also converts the code format of the reproduced signal into a bipolar format according to the partial response system, and supplies a bipolar signal to an automatic threshold control circuit. The automatic threshold control circuit converts the output bipolar signal of the equalizer into a unipolar signal, by converting levels "+1" and "-1" of the bipolar signal into a level "+1" of the unipolar signal and maintaining a level "0" of the bipolar signal as a level "0" of the unipolar signal. Even if the level of the reproduced signal fluctuates when the signal level is being discriminated, the automatic threshold control circuit generates a reference signal (control voltage) which has a suitable time constant and follows the level fluctuation, so that the level comparison is carried out with the threshold values set to optimum values. The automatic threshold control circuit thus produces a digital signal which has been restored into the original unipolar signal which was obtained at the time of the recording.
When reproducing the recorded signal from the magnetic tape by the magnetic head, deterioration is sometimes introduced in the high frequency components of the reproduced signal and the amplitude of the reproduced signal decreases due to signal dropouts, as is well known. There are two major causes for such signal dropouts. A temporary or instantaneous signal dropout in which the level of the reproduced signal decreases for a relatively short period of time, is caused by foreign particles such as dust particles which are adhered on the magnetic layer. A permanent signal dropout in which the level of the reproduced signal decreases for a long period of time, is caused by the unevenly coated magnetic layer and the scratches on the magnetic layer.
Hence, because of the signal dropouts described above, the level fluctuations in the reproduced signal due to causes such as instability of the tape travel, and noise which is generated from the reproducing amplifier, the envelope of the reproduced signal accompanies level fluctuations with a long period, and the level of the sound voltage fluctuates. Furthermore, the amplitude of the reproduced signal decreases greatly due to the signal dropout. The period in which the amplitude of the reproduced signal decreases due to the signal dropout, is determined by the physical size of the dust particles and scratches on the magnetic layer, or the physical size of the dust particles on the surface of the head gap in the magnetic head, and the traveling speed of the magnetic tape. In some cases where the signal dropout is caused by minute dust particles, the level of the reproduced signal may decrease in a period which corresponds to one bit.
In the conventional magnetic recording and reproducing apparatus described heretofore, the automatic threshold control circuit included a pair of comparators. The bipolar signal was supplied to one input of each of these two comparators. Further, a reference signal which was obtained by subjecting the bipolar signal to a full-wave rectification and then passing the full-wave rectified signal through a smoothing circuit which has a time constant determined by a capacitor and a resistor, was supplied to the other input of each of these two comparators. Accordingly, it was possible for the automatic threshold control circuit to follow the effects of the signal dropout which caused the amplitude of the reproduced signal to decrease for a period which was sufficiently long with respect to the maximum repetition frequency (2-bit transmission period) of the reproduced signal. However, there was a problem in that the automatic threshold control circuit could not follow the effects of the instantaneous signal dropout which occurred within 1-bit transmission period.
In addition, the waveform equalization which was carried out in the equalizer was fixed regardless of the amplitude of the reproduced signal. Thus, when the signal dropout occurred, and also at the time of steady amplitude reproduction, the level comparison between the input reference signal and the input bipolar signal could not be carried out in a normal manner in the two comparators described before. This was mainly because the magnetic recording and reproducing frequency characteristics were inconsistent due to the magnetic material used for the magnetic tape, or because the reproducing frequency characteristics of the magnetic head was inconsistent in a multi-track magnetic recording and reproducing apparatus which simultaneously carries out recording and reproduction with respect to a plurality of tracks on the magnetic tape. This was one of the reasons why the code error occurred upon level discrimination in the automatic threshold control circuit.
Further, in the automatic threshold control circuit described before, the reference signal was generated from a reference signal generator within the automatic threshold control circuit. This reference signal generator was made up from a pair of switches which are supplied with positive and inverted phase outputs of a differential amplifier of the automatic threshold control circuit and are controlled of their open and closed states by a clock signal, a pair of diodes which rectify outputs of the switches, and a smoothing circuit comprising a capacitor and a resistor for smoothening the rectified output of the diodes. However, as is well known, the diodes have a non-linear forward voltage versus current characteristic, and a region which may be considered substantially linear is only in a range under 0.6 volts. Accordingly, when the level of the reproduced signal was adjusted so that the reference signal is obtained in the substantially linear region of the diodes, it became difficult to carry out the level comparison in level comparators with a satisfactory signal-to-noise (S/N) ratio. On the other hand, when the level of the reproduced signal was adjusted so that the reference signal is obtained in the non-linear region of the diodes, the voltage level of the reference signal which was obtained included the drops in the forward voltages of the diodes. Thus, this meant that an error is introduced in the value of the reference signal with respect to the input bipolar signal, and it was impossible to carry out an accurate level comparison.
In addition, in the multi-track magnetic recording and reproducing apparatus which simultaneously carries out the recording and reproduction with respect to a plurality of tracks which are formed parallel to each other in the longitudinal direction of the magnetic tape, for example, the error rate of the signals which are reproduced from the tracks is sensitive to the differences in the recording and reproducing characteristics of the tracks and to the differences in the characteristic of the head in the apparatus which actually carried out the recording and the characteristic of the head in a different apparatus which carries out the reproduction. Therefore, the levels of the reproduced signals had to be adjusted with extreme care. Such adjustment of the levels of the reproduced signal, may be carried out by adjusting the gain of an amplifier which is located in an input stage of the equalizer, for example.
Further, in the conventional magnetic recording and reproducing apparatus described heretofore, a smoothing capacitor having a large capacitance was required within the reference signal generator. Moreover, the pair of switches in the reference signal generator had to be high precision switches, but such high precision switches were difficult to make into an integrated circuit (IC). Hence, there was a problem in that the circuit construction of the reference signal generator was unfit for realization in the form of an integrated circuit.